1998-02-23

Comparison of Vehicle Running Loss Evaporative Emissions Using Point Source and Enclosure Measurement Techniques 980403

Two test techniques are currently used for the measurement of evaporative hydrocarbon emissions during the running loss portion of the Enhanced Evaporative Emission Test Sequence: point source and enclosure. The point source technique uses a constant volume sampling (CVS) system to sample and measure running loss emissions at the vehicle's discrete vapor vents. The enclosure technique uses atmospheric sampling within a sealed housing for evaporative determination (SHED) to measure running loss emissions.
A test program was carried out to compare the two measurement techniques and investigate the differences between the measurements. During the test program, point source emissions averaged 0.001 grams/mile (0.0006 grams/km), or 2% of the standard (0.05 grams/mile or 0.031 grams/km), while enclosure emissions averaged 0.021 grams/mile (0.013 grams/km), or 41% of the standard. Running loss enclosure samples were speciated to examine possible causes for the enclosure emissions being higher than the point source emissions. This analysis indicated that the majority of the running loss enclosure hydrocarbon emissions were fuel-based. Exhaust emissions and background hydrocarbon emissions from the vehicle and test equipment also contributed to enclosure emissions. Hot soak and diurnal emissions were speciated for the same vehicles, and though the results were similar, the species present in each test were not present in the same ratios. This indicates that the hydrocarbon emissions measured in evaporative emission tests are weighted by the different mechanisms effective in each test.

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